This invention relates to a multicolor thermal transfer copying system, and more particularly to a multicolor thermal transfer copying system having a paper feeding mechanism which avoids color shear in printing.
For example, a multicolor thermal transfer copying system is used for the purpose of obtaining a hard color copy from a picture information of a figure etc. which is indicated in a Braun tube display of many kinds of devices. A copying system of this type has yellow, magenta, cyan and black thermal head members arranged along the feeding route of the copying paper which is fed at a constant speed. When the copying paper passes through each thermal head member, a heat current for transferring is supplied to the thermal head which is in pressure contact with the copying paper via the ink ribbon, whereby a desired color ink which is coated on the ink ribbon is transferred to the copying paper to obtain a multicolor picture. Accordingly, the copying paper feeding speed must be controlled extremely accurately, and the time that a certain point on the copying paper reaches the next thermal head member after passing through one thermal head member must be precisely maintained, in order to obtain the desired multicolor picture by successively printing the same picture in a different color at each thermal head member.
In such a system, each thermal head is driven successively with a time delay which is decided by the distance between each thermal head member and the copying paper feeding speed.
The copying paper feeding is achieved by rotating the platen which is in pressure contact with the thermal head via the copying paper and the ink ribbon. The platen surface is made of soft rubber member. The copying paper and the ink ribbon which contacts the copying paper are fed according to the rotation of the platen by frictionally contacting the rubber member to the copying paper. Therefore, the feeding speed of the copying paper is determined by the rotating speed of the platen at each thermal head member. Actually, however, since the platen surface is made of soft rubber as mentioned above, it is difficult to make the outer diameter of the platen precise predetermined dimension. As a result a color shear in printing occurs because the paper feeding speed differs due to the difference of the outer diameters of the platens, even if the platens at each thermal head member are rotationally driven at the same rotating speed by a single motor. To avoid such drawbacks, it is necessary to use platens of nearly the same outer diameter which need be selected from a group of platens of slightly varying dimensions. It is, however, difficult to select platens of the same outer diameter in order to satisfy the required accuracy since the size of the thermal head heating element is about 100 .mu.m.times.100 .mu.m.